Feasibility of making sound power measurements in the NASA Langley V/STOL tunnel test section

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r NASA TECHNICAL NASA TM X- 73934 MEMORANDUM z FEASIBILITY OF MAKING SOUND POWER MEASUREMENTS IN THE NASA LANGLEY V/STOL TUNNEL TEST SECTION by Thomas F. Bronks, James Scheiman, and Richard J.,Silcox } A (NASA-TM-X-73934) FEASIBILITY OF MAKING SOUND POWER

MEAS N76-3x0029

UREMENTS IN THE NASA LANGLFY V/STOL TUNNEL TEST SECTION (NASA) p HC CSCL 20A Uncias G3/71 48414 This informal documentation medium is used to provide accelerated or special release of technical information to selected`users.

-The contents may not meet NASA formal editing and publication standards, may be re- r wised, or may be incorporated' in another publication.

4 t ^^17 3 r /^ ^^kJt^r0.t ^ n - National Aeronautics and Space Administration Langley Research Center : fidASA 5^' r,: Hampton, Virginia 2,8 1,365 ^' wa owa

1. Report No, 2. Government Accession No. 3. Recipient's Catalog No.

NASA TM X-73934 4. Title and SuWitle 5. Report Dete

1V

Feasibility of Making Sound Power Measurements in the 6. Performng or izetionCode NASA La>.gley V/STOL Tunnel Test Section 7. Author(s) S. Performing Organization Report No.

Thomas F. Brooks, James Scheiman and Richard J.Silcox 10. Work Unit No.

9. Performing Organization Name and Address 505-06-23-02 Langley Research Center 11. Contract or Grant No.

Hampton, VA 23665 13. Type of Report and Period Covered 12. Sponsoring Agency Name and Address Technical Memorandum National Aeronautics & Space Administration 14. Sponsoring Agency code Washington, DC 20546 15. Supplementary Notes 16. Abstract t Based on exploratory acoustic measurements in Langley's V/STOL wind tunnel, recommendations are made on the methodology for making sound power During measurements of aircraft components in the closed tunnel test section.

airflow, tunnel self-noise and microphone flow-induced noise placerestrictions Models of on the amplitude and spectrum of the sound source to be measured.

aircraft components with high sound level sources, such . as thrust engines and powered lift systems, seem likely candidates for acoustic testing.

17. Key Words (Suggested by Author(s)) (STAR category underlined) 18. Distribution Statement Acoustic power Wind tunnel acoustic measurement Unclassified - Unlimited 19, Security Clusif. (of this report) Price- 20. Security ClPoif. (of this pails) 21. No. of Pages 22.

Unclassified 38 $3.25 Unclassified

FEASIBILITY OF MAKING SOUND POWER MEASUREMENTS IN THE NASA LANGLEY V/STOL TUNNEL TEST SECTION By

INTRODUCTION Aeroacoustic data obtained from aircraft models in wind tunnels can be

valuable because the effect of forward flight can be simulated and many _noise

associated aerodynamic parameters can be controlled and/or determined. In recent years, small open jet wind tunnels which are particularly suited for noise measurements have been constructed. For the larger tunnel test sections, which are required for more realistic model sizes and Reynolds numbers, the general trend has been to modify existing facilities rather than construct new large facilities. This approach has resulted from a compromise of cost and quality of ,data.

Most existing wind tunnels were not designed to be used for acoustical measurements. Because of this, there are two major problem areas that affect the quality of acoustic measurements in most existing tunnels high background noise, which masks the model noise signal, and acoustic environment complexity, which-affects the data interpretation., Examples of unwanted sources of background noiseare boundary layer turb lence, wall surface vibrations, open jet mixing (for open jet tunnels), tunnel' drive fans, impingement on the, flow collector, sting for support of model, and auxiliary tunnel equipment such as compressors and pumps. When acoustic measure- ments are made using microphones placed in the airstream, microphone wind noise (or flow-induced noise) adds to the background noiselevels.

The quality of the acoustical field , is also affected by the condition of wall surfaces. If the wall boundaries have widely differing sound absorption - characteristics, the sound field can be very complex. The type of wall surface t

needed for a quality acoustic field depends on the type of measurement to be performed. If sound power measurements are required, then highly reflective

wall surfaces are

wall boundaries are desirable. Highly sound absorbing desirable when direct free-field measurements and model directionality are needed.

Reference I reports the progress made at the NASA Lewis 2.74 x 4.56 m The Lewis tunnel employs V/STOL wind tunnel in making acoustic measurements.

acoustic mufflers, baffles, and wall treatment to reduce sound transmission f and reflection.

Acoustic calibrations have been performed in other wind tunnels to determine the suitability of making noise measurements, e.g., see references 2, 3, 4, and 5. In addition to these calibrations, studies have been made to determine methods for improving tunnel acoustic characteristics, e.g., see references 6, 7, and 8.

Acoustic.testing in the NASA Langley V /STOL wind.tunnel is the subject of this report and three previous studies, references 4, 5, and 6.- In this facility, aircraft takeoff, landing, and low-subsonic cruise speeds may be simulated.

t The test section is 6.6 m wide and 4.4 m high and the maximum speed capability is 103 m/sec. The test section can be operated in an open or closed configuration.

In the open configuration, the ceiling and ,walls of the test section enclosure are lifted above the airstream. The airstream is then surrounded by stationary air in the chamber (large room) enclosing the test section and the model preparation area.

Reference 4 considers the acoustic signal-to-noise ratio of model-produced noise above tunnel and microphone flow'-noise in the V/STOL tunnel. Data are -2- z '

presented which describe the tunnel background noise and airframe noise from an 1{' unpowered 1/25-scale model of the Boeing 747-200. Data were obtained for the both its openand closed configurations. The results of tunnel tesu section in the tests (ref. 4) were disappointing because the airframe noise produced by the 8-foot span model could not be detected adequately in this tunnel at model-to microphone distances of 5 and 15 feet. Because of the background noise, the resolution was not adequate to allow a good description of the model airframe ' p p noise spectrum shape, even , the peak-level frequencies.

Reference 5 dealt mainly with the acoustic environment regarding sound in power measurements the open test chamber. This study identified various tunnel noise sources. it also determined that the acoustic field of the open test chamber was semi-reverberant and very complex. !Because of this, sound power determination by acoustic measurements in the open chamber reverberant field was discouraged. In addition, determination of sound power by measuring the intensity of the direct sound was investigated. It was concluded that this method would be feasible if measurement microphones could be placed close enough to the modelto avoid the open chamber reverberant field. This reference also considered sound power determination in the reverberant field of the closed test section.

In reference 6, methods to reduce .unwanted reflections and reverberation in the open tunnel chamber were studied using a scale model of the V/STOL tunnel.

Some success was found for reducing reflection, thereby allowing a somewhat larger distance between the noise source and microphone, by lining certain strategically located surfaces with sound absorbing material.

The purpose of this report is to evaluate the feasibility and propose a t _ i

method for making sound power measurements of model aircraft noise sources in the V/STOL wind tunnel. This report presents acoustic data taken in and near -flow the test section in the open and closed configuration for the flow and zero cases. Tests were conducted with standard microphones and limited use of a porous surface microphone (ref. 9). Results for the porous surface microphone are presented in the appendix. Specific recommendations are given for the methodology of sound power determination to use for the closed test section k configuration. The feasibility of acoustic testing is found to depend on the relative spectral shape and level of the source to be tested compared to that of the tunnel's and microphone's induced-noise during flow.

APPARATUS AND METHODS Acoustic tests were performed to determine the feasibility of making sound the NASA Langley V/STOL wind tunnel. The tests utilized power measurements in two separate noise sources. One source with broadband output was used to the closed determine the uniformity of spatial distribution of sound pressure in section in the absence of tunnel airflow. The other source was a turbine engine simulator producing noise with prominent pure tones. This source was tested with and without airflow in both the open and closed tunnel configurations.

_ Facility

A schematic of the NASA Langley Research Center V/STOL wind tunnel is

presented in figure 1. It is a closed circuit tunnel whose closed loop is. 234 m (770 ft) in length. The air is driven by electrical powered fan which has an nine blades and is 12.2 m (40 ft) in diameter. The fan can provide airspeeds -4-

f fan speed of 275 rpm.

up to 103 m/sec (338 ft /sec) in the test section with a either the open or closed The tunnel test section can be used in configuration. The closed test section is 6.6 m wide (21.7 ft), 4.4 m (14.5 ft) { !

high and about 21.3 (70 ft) long and is fully enclosed by hard walls. In the open configuration, the ceiling is raised to a height of 7.5 m(24.5 ft) above chamber floor. This is about 3.0 m (10 ft) above the flow stream.

the test z The side walls are extended above the raised ceiling. In the open configuration the airstream becomes a free jet on the three open sides and is surrounded by stationary air in the chamber enclosing the test section, see figure 1.

The test section is equipped with a boundary layer suction fan and a ground belt which is used to simulate takeoff and landing conditions. The fan was not operated andthe belt was removed for the series of tests reported `- k herein.

r Noise Sources One noise source was a standard centrifugal fan reference source described in references 10 and 11. This broadband source is approximately omnidirectional and is used often for sound power calibration, especially in many industrial noise measurements.

The second noise source consisted of two 14 cm (5.5 in`) diameter tip turbine <' fan engine simulators. These simulators were driven by compressed air actuated tip tubines which in turn drive the blades providing primary engine thrust.

These simulators are commercially available and commonly used for powering'wind u tunnel models. These two simulators were mounted on an available wind tunnel odel (1/12- scale Gulf Stream II with an 2..13 m (7 ft) wing span), a description { -5-

of which may be found in reference 12. This source offered a high signal-to- noise ratio over a large frequency range, thus ,allowing a noise distribution comparison between the open chamber and closed test section. A photograph of independent this model can be seen in figure 2. The simulator thrust and rpm was of tunnel speed.

Instrumentation and Test Procedure acoustic data was reduced in real time and recorded on an X-Y plotter.

The For the data presented in this report, the real time analyzer was operatedwith a bandwidth of 120 Hz on a continuous frequency sweep. The dynamic range was 54 dB.

The microphones used for most of the test were standard 3.17 mm (1/8 inch) pressure microphones with noise cones (references 1,3, 14, and 15). Microphone p positions 1, 2, 3, and 4 and their mountings are shown in figure 2 and the microphone locations are shown in figure 3. The microphones were mounted on stands which were bolted to the tunnel floor. For the closed test section, microphone positions 1, 2, 3, and 4 were used. In the open chamber, microphone positions 1, 2, 5, and 6 were used. Positions 5 and 6 were located outside the free jet shear layer.

In one series of runs, for the closed test section, a porous surface microphone system (ref. 9) was mounted at position 2. Results of these measurements are given in the appendix: Tests with the broadband noise source were conducted at zero tunnel speed with the walls in the closed configuration only: The tests with the simulators were conducted with various thrust settings -6-

and forward speeds. The thrust settings were at 0,266.9, 489.3, and 667.2 N I, corresponds to blade passing frequencies of 0, 5430, (0, 60, 110, 150 lbs) which ' 7350,,, and 8730 Hz. Tests were conducted at tunnel velocities of 0, 15.2,.23.4, 34.4, and 50.2 m/sec (0, 50, 77, 113, and 165 ft/sec). Both open and closed configurations were employed in this series of tests. A detailed listing of the test conditions is shown in Table I l RESULTS AND DISCUSSION Acoustic tests were performed to determine the feasibility of making sound power measurements in NASA Langley V/STOL wind tunnel. The test incorporated two separate noise sources. One source with broadband output was used to determine the uniformity of spatial distribution of sound pressure in the closed section in the absence of tunnel airflow. The other source was a turbine engine simulator producing noise with prominent tones. This source was tested with and without airflow in both the open and closed tunnel configurations Of interest during the tests was the unwanted tunnel and microphone flow-noise generation as well as spatial distribution of sound from the source, Broadband Noise Source Test For the broadband noise spatial distribution test, the broadband noise source was placed beside the model about one meter forward of microphone location 3 (see fig. 2).

The results are plotted in figure 4 which shows the noise spectra recorded at the different microphone positions. Also shown in the figure is the spectrum r of the reference source measured in a reverberation chamber. The reverberation chamber data was obtained using a'bandwidth of 20 Hz, while the tunnel data -7-

presented in figure 4 and the remainder of the report was analyzed at bandwidths of 120 Hz.

The spectrum from microphone position 3 is presented in figure 4 as a point of interest. Microphone position 3 was in the direct and near acoustic fields of the source. The fact that the source is not truly omnidirectional is indicated by the relatively "lumpy" shape of the spectrum for this direct field measurement. It is also seen that the relative spectral levels between position 3 and the other positions indicates a rather intense reverberant field in the closed test section especially in the upstream direction (positions 1 and 2).

The fact that the levels are not generally the same for microphone positions 1, 2, and 4 indicates that the acoustic field is not diffuse. However, the spectral shapes measured at these positions do not deviate greatly from the spectral shape found in the reverberation chamber. These deviations, which are not excessively irregular, may be accounted for by proper calibration. This no-flow information indicates that sound power measurements may be made under conditions for the closed configuration.

Engine Simulator Noise Tests The simulators were used as a source of noise with tonal content over a large frequency range. This source offered high signal-to-noise ratio thus' allowing microphone signal comparison between the open and closed tunnel configuration.

Closed test section configuration. - For the four microphones in the closed x test section, figures 5, 6, 7, and 8 show the background noise spectra as a function of tunnel flow velocity while engine thrust is zero. This background noise increases as tunnel speed increases. The background noise is -8-

a combination of tunnel flow noise and microphone wind noise. For comparison purposes, the figures also show the spectra of the engines for two test conditions where tunnel velocity is zero.

For microphone positions 1 and 2, upstream in the test section, the flow noise is about equal for the same tunnel velocities. For the downstream microphone positions 3 and 4 the flow-noise is increased above that of the upstream microphone positions. Microphone wind noise due to incident turbulence is responsible for the increased levels. It is expected that positioning of the downstream microphones to avoid the tunnel boundary layer and any wake from the aircraft model would result in significant reductions in this self-noise generation.

Figures 9 and 10 are for an engine thrust of 23.4 N and for a tunnel velocity of 0 and 50.2'm,/sec, respectively.

Figure 9 shows good definition for the tonal content in the spectra throughout the 40 kHz frequency range. The apparent spectral "smearing" which results in a loss of detail in the higher frequency range for the downstream microphone position 4 is a characteristic of tones propagating through the turbulent engine exhaust. Exhaust turbulence not only produces noise but also causes Doppler shifts in the simulator turbine noise spectrum which spreads acoustic energy over a broader frequncy range.

The effect on the noise spectra due to a tunnel flow velocity of 50.2 m/sec may be seen by comparing figures 9 and 10. For frequencies below about two kHz for the spectra shown, flow-induced noise masks out the source signal For frequencies above a few kHz, signal quality is maintained as evidenced by the relative high noise intensity of the source compared to the flow noise in the -g_

closed test section as was expected from the results shown in figures 5, 6, 7, and 8.

Open test chamber configuration. - Two microphones were placed at positions 5 and 6 (see figure 3), outside the free jet region, in the preparation area of the test chamber. Microphone positions I and 2 were maintained. The spectra for these four microphone locations for two different tunnel speeds are shown in figures 11 and 12.

Comparing figure 11 to figure 9, for the no-flow case, it is seen that one effect of raising the walls to create the open test chamber configuration is to reduce the noise spectral level at positions 1 and 2 on the order of 10 dB.

Also, large spectral differences are noticed between the four microphone positions, indicating a very non-diffuse acoustic field.

Figure 12 shows the effect of a tunnel flow velocity of 50.2 m/sec on the noise spectrum for the open configuration. It is seen that the effect of flow is to diminish the quality of the acoustic signal at the microphones, as evidenced by substantial spectral "smearing" in figure 12 compared to figure 11.

This effect is found more severe for the open than for the closed configuration (figures 9 and 10).

The out-of-flow microphones, positions 5 and 6, experienced lower background noise than the in-flow microphones, postions 1 and 2. This is because microphone wind noise was eliminated and only the tunnel flow-induced noise was perceived by the out-of-flow microphones. It is interesting to note that the in-flow microphones, positions 1 and 2, experienced increased background noise compared to the same microphones in the closed configuration for the same tunnel speed.

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FEASIBILITY AND METHODS OF ACOUSTIC POWER MEASUREMENTS

Various methodologies of sound power determination are reviewed in

reference 11. For use in the VSTOL tunnel, the method chosen should be of predictable accuracy and be versatile enough to accommodate various test conditions and model configurations. 'Tunnel flow background noise, which

reduces the signal-to-noise ratio, and the

tunnel's acoustic field environment

place restrictions on the choice of methods. Both factors, background noise and the acoustic field, are found to be dependent on tunnel configuration.

Test results indicate that the closed wall configuration is highly reverberant.

In this configuration, good acoustic signal resolution (or signal- to-noise ratio) is found for the microphones in the tunnel airflow in frequency

ranges where

the flow-induced noise does not "mask" the source noise.

;.

For the open configuration, lower flow-induced background noise is found for the microphones placed out of the flow.

Microphones remaining in the flow

experience an

increase in background noise compared to when the wall

configuration is closed.

for al li microphones in the

open wall configuration tests (none

placed in the shear layer between the flow stream and stationary air), the amplitude and resolution of the source noise during airflow is diminished compared to when the wall configuration is closed.

Open Tunnel Configuration The tests, performed in the open tunnel configuration, used microphone positions in the semi-reverberant field of the test chamber.

The diminished signal-to-noise ratio, compared to the closed configuration, serves to reduce

the value of the open configuration for making acoustic measurements.

In the study of reference 5, no attempt was made to measure a noise source during tunnel flow, so no mention was made of a signal resolution problem.

Stull reference 5 did not recommend acoustic measurements in the open configu- ration unless measurements were made in the direct field of the source, to avoid the semi-reverberant field. Reference 6 determined that the radius of

the direct field (or free field) could be extended somewhat by the application

of sound absorbing materials to the floor and raised ceiling of the open tunnel'.

Direct field measurements to determine sound power as suggested in reference 5 is a method to avoid the problems of acoustic data interpretation associated with thecomplex semi reverberant field. Successful implementation of this method would also allow evaluation of sound source directionality.

However, requirements for the success of this method are restrictive. Measure- ments must be made on a radius about the source which is in the direct field (or free field) and also the far field of the noise source for the frequency range considered. This requirement is considered to be met when the measurement radius is within a region where doubling the measurement distance from the source would result in a six decibel drop in sound pressure level for all directions from the actual source to be tested (e.g., see reference 11). For example, the above requirements were met in tests where directionality measure- ments were made on small sources in the NASA Lewis V/STOL wind tunnel (see ref. 1).

However, meeting the requirements of the direct measurement method may be difficult,if not impossible,for larger source configurations. Far field conditions exist roughly at 2 or 3 times the largest linear dimension of a -12-

--- ---- l noise source. For many aircraft model configurations, mounted with a noise- producin g engine, the length or span of the model must be considered the applicable source dimension and not a dimension of the engine. This would be true where substantial aircraft wing or body shielding, diffracting or reflection, effects are present. In such cases,it is likely there would be no measurement radius at which the direct field method would be accurate.

An additional restriction concerning the signal-to-noise ratio problem is that microphones should not be placed near or in the shear layer between the flow stream and the stationary air. Because of the excessive turbulence in - this region, microphone wind noise would likely be unacceptably intense.

To improve the quality of direct measurements, a directional microphone system could be useful in some applications. See Appendix Closed Tunnel Configuration Results obtained in this study suggest that good sound power measurements of certain noise sources can be made in the V/STOL tunnel facility in the closed configuration. Aircraft components with high sound level sources, such as thrust engines and powered lift systems, seem likely candidates for acoustic testing.

The use of measurement positions in the closed configuration upstream and downstream of a-source takes advantage of the relatively good signal to noise ratio and uniformity of the sound field compared to that of the open configuration.

Recommended sound power -meas rement_ u procedures. - In reference 1, where reverberant field measurements were conducted in the NASA Lewis Research Center V/STOL facility, it was assumed that the sound field. in the tunnel could be 13-

were used to connect treated as an essentially diffuse field. Factors which sound pressure measurements to sound power were computed from reverberation time measurements and general tunnel geometric properties.

the following alternate approach, Rather than the method used in reference 1 which is based on the comparison method, is recommended. The closed circuit tunnel is recognized as a large rectangular duct with _variable cross-section and complex absorption and reflection.

Choose two tunnel cross-sections,-one upstream and one downstream from the location of the sound source. Sound power produced by the source can be determined from the sound pressure measurements over thecross-sectional areas represented by AU for the upstream position and by for the downstream Ad position. The total sound generated is: power u U +LFW+LFdWd W=Wu+Wd or W _ [1 + LFu ] Wu + [1 + LFd ] WU (1) where for i=u (upstream) or d (downstream) Pmi P2 Wi - A

f dA - (2)

PC Pc = average mean-square sound pressure over area A i and frequency P2 mi bandwidth of interest,

1.

characteristic impedance of air (value subject to temperature and pc F humidity conditions), a function of LF. = loss factors i`or measurement area A., which is frequency.

The loss factors as defined here should account for deviation from the ideal case where all power passes through the cross-sections without reflection.

Effects that may cause deviation from the ideal include: presence of non- i wave patterns in the tunnel sections, which may be propagating standing affected by the source directionality; absorption of sound through the tunnel walls; and any feedback sound from the turning vanes, etc.

For an omnidirectional source in a large duct without flow the following relation is valid: - LFu I [1 LFdI Wd Wu = + [1 + (3) i.e., equal power is radiated from source towards A u and Aa.

The loss factors are found by experiment using an ominidirectional and a directional reference source. Any change value pc for the tunnel air in the may be accounted for by using equation (2). After calibrating the reference !

sources for sound power as a function of frequency in a standard reverberation = room, the reference sources should be tested in the test section at various model noise source locations. For the reference sources, determination of Wu ?

and Wd ,knowledge of W from reverberation room tests.,and use of equation (3) would allow the calculation of the loss factors as a function of frequency and r _15_..

bandwidth.

Both a di rectional and an omnidirectional reference source should be u tested so that any variation of the experimentally determined values of the loss factors will provide an indication of the expected measurement accuracy This is especially when sources of unknown di recti onal i ti es are tested.

important since mean tunnel flow can affect the source directionality.

The calibration would be conducted for the tunnel without flow. For tests

involving non-zero tunnel flow, the results and subsequent application of

equations l and 2 should contain correction factors. is because the This relationship between acoustic energy flux and acoustic pressure at the measurement locations is affected by the presence of mean flow. The corrections are derivable with the use of conservation of acoustic energy principles, e.g., see reference 16.

However, for many test conditions the corrections would be minimal. It can be shown, for a noise source that is not highly directional, that with a tunnel flow Mach number, M , equation (1) would underestimate W on the order of [l + M 2 ], e.g., this is an error of 0.2 dB for M = 0.2.

Individually by using T equation (2), for any source, where M Au is measured at the respective areas and Ad: Wu would be dverestimated on the order of [1/(l - 2M + M 2 )], or 2 dB for M = 0.2; Wd would be underestimated on the orderof (l + 2M + M 2), or 1.6 dB for M = 0.2.

Sound absorption in the wall boundary layers is not taken into account in the recommended calibration procedures. It is assumed that this _effect is small except for higher frequencies.

The main consequence of the boundary layer presence is believed to be a spreading (or smearing) of the acoustic energy over -16- OF T111, REPRODUCIBILITY h3 POOR, ORIONA.L PAGE

Referring to figure 10 for the particular microphone the noise spectrum.

is seen that the downstream microphone locations chosen for this study, it position is more affected by spectral smearing than the upstream microphones.

The microphones recommended are the standard condenser type with nose cone.

characteristic (improved by the nose cones) make them Their omnidirectional valuable for reverberant measurements. The diameter of the microphones used The smaller dia- should be determined by the frequency range requirements.

meters give greater frequency amplitude linearity while larger diameters give less wind noise, e.g., see reference 14 and 15.

The flow induced noise from the microphone mounting system can be reduced by streamlining the microphone mount and careful selection of microphone boundary positions to avoid model turbulent wake region and tunnel layer.

The measurement areas, i.e., A and A and the microphone positions with- in the areas should be chosen to maximize source resolution and to minimize background flow noise. Microphones may be placed between, but not close to Also, the upstream diffusing screen and downstream turning vane (see,fig. 1).

positions close to the source should be avoided to minimize any effect of source In practice it is recommended that up to six microphones be positioned along each of the forward and aft cross-sections chosen for measurement. The larger the number of microphones, the smaller the error would be in the determination of the average-mean-square sound pressure over the respective be cross-sectional areas. Also, it should noted that a large number of microphones would allow tests to continue even with an unacceptable signal from one or two of the microphones.

... -.. ^ _. .- ... ... ........ ^..r.`R' A primary advantage of this method is that the various effects of absorption, a reflection etc., are lumped together in one factor which is determined by experiment in the tunnel calibration study. The calibration need not be repeated as long as the tunnel is not altered and the position of the micro- phones remain unchanged. This fact would permit a rapid and systematic acquisition and analysis of acoustic data. The acoustic data could be stored on tape and the data could be analyzed by a computer program incorporating the calibration information and a signal analysis subroutine.

f 18_

CONCLUDING REMARKS A feasibility study of making sound power measurements in the NASA V/STOL tunnel has been conducted for the tunnel in both its open and closed (wall) Such measurements are found to be feasible for the closed configurations.

closed configuration has tunnel case for sources with high noise levels. The levels of been found superior to that of the open in regards to the relative Also the source signal compared to tunnel and microphone background noise.

field for the closed configuration is determined to be highly acoustic reverberant and fairly diffuse whereas that of the open is less reverberant and more complex.

For the open tunnel configuration, acoustic measurements in the direct and free field of small sources should be possible.

_ The recommended method of sound power measurement for the closed configuration -uses standard microphones, with nose cones, upstream and downstream of the source at cross-sections in the reverberant field. With proper calibration the above method should be of predictable accuracy and accommodate various test conditions and model sizes.

19-

Measurement of Model Propulsion Diedrick, J. H.; and Luidens, R. W.: 1.

System Noise in a Low-Speed Wind Tunnel, Paper 76-91, AIAA 14th Aerospace Conf., Washington, D.C., Jan. 26-28, 1976.

'on of the Feasibility of Making Model Acoustic A.: Investigate Y Bees, D.

2.

Measurements in NASA Ames 40 by 80 foot Wind Tunnel. NASA CR-114352.

Bolt Beranek and Newman Inc. 1971.

Acoustic Characteristics 3._ Abrahamson, A. L.: Kasper, P. K.: and Pappa, R. S.: of the NASA - Langley Full-Scale Wind Tunnel Test Section.

NASA CR-132604, 1975.

4.

McBride, J. F.; and Burggraf, W. D.: Results of Acoustics Testing in the the NASA-Langley V/STOL Tunnel.

Rep. D6-43297, The Boeing Company,Commercial Airplane Division, June 1974.

5. V6r, I. L.: Acoustic Evaluation of the NASA-Langley V/STOL Wind Tunnel.

.Rep..

2288, Bolt Beranek and Newman Inc., Dec.

1971.

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L.; Anderson, D. W.; and Bies, D. A.: Acoustic, Modeling Study of the Open Section of the NASA Langley V/STOL Wind Tunnel.

Rep. 31`79, Bolt Beranek and Newman Inc.., Oct. 1975.

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8. Piersol, A. G.: and Rentz, P. E.: Utilization and Enhancement of the NASA-Lewis V/STOL Wind Tunnel for Inlet Noise Research, Rep. 2743, Bolt Beranek and Newman Inc., May 1974.

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9. Noiseux, D. U.; Noiseux, N. B.; and Kadman, Y.: Development, Fabrication And Calibration of a Porous Surface Microphone in an Aerofoil. NASA- CR-132C36. Bolt Beranek and Newman Inc., 1975.

Baade, P. K.: Standardization of Machinery Sound Measurement. ASME Publ.

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69-WA/FE-30, The American Society of Mechanical Engineers, 1969.

11. Beranek, L. L.: Noise and Vibration Control. McGraw-Hill Book Company, c. 1971.

-.12. Hoad, D. R.: Longitudinal Aerodynamics of a Low-Wing-Lift-Fan Transport Including Hover Characteristics In and Out of Ground Effect. NASA TM X-72670, 1975.

13. Anon: Instructions and Applications, Condenser Microphones, type 4144, 4145, 4146, and 4161. Bruel and Kjaer Instruments, Inc. Cleveland, Ohio, 1973.

14. Rasmussen, G.: Measurement Microphones. Proceedings 1972 International Conference on Noise Control , En ineerin Washington, D.C., Oct. 4-6, 1972, Engineering, pp. 387-392., 15.

Oswald, L. J.: The Wind Noise of Nose-Cone-Protected Microphones.

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16. Candel, S. M.

Acoustic Conservation Principles and an Application to Plane and Modal Propagation in Nozzles and Diffusers. J. Sound and Vib ,, vol. 41, no. 2, 1975, pp. 207-232.

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APPENDIX POROUS SURFACE MICROPHONE The porous surface microphone system (reference 9) or some other directional microphone could be used to advantage in some applications. Proper use would tend to improve the direct measurements by essentially extending the effective direct acoustic field of the source for higher frequencies.

The porous surface microphone system which i-ncorporates a standard 1.27 cm microphone was tested so that its response in a reverberant field could be compared to the standard microphone. Using the engine simulators operating at 489.3 N thrust as the noise source, microphone position 2 (see fig. 3) was used alternately as the measurement location for both the standard and porous surface microphone. Both microphones were pointed in the upstream direction. The microphone at posi:tion 1 was used as a reference to verify that the conditions of the test were the same. Figure 13 shows the spe.ctral comparison for the closed tunnel configuration and no flow.

For the frequency range shown in figure 13 the standard 3.17 mm (1/8 in) is known to have a flat frequency response, pressure microphone with nose cone see references 14 and 15. Thus the spectrum shown for the standard microphone is considered to be the true sound pressure level spectrum for the sound at from the porous surface microphone deviates position 2. The spectrum rendered microphone is a directional system (details from the true spectrum because the They are different because the acoustic of which are given in reference 9).

comprised of a combination of radial and pressure at a measurement point is

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